1. Field of the Invention
This invention relates to a method of transmitting light having a substantial UV component via an optical fiber, and especially to a method of transmitting light of wavelengths in the vacuum UV.
2. Introduction to the Invention
Although conventional fiber optics systems, such as are coming into increasing use in the telecommunications industry, employ light in the visible or, more usually, in the near-IR wavelength range, it is sometimes desirable to be able to transmit UV light via an optical fiber. By UV light is meant light having a wavelength between about 150 and 400 nanometers, with that portion having a wavelength less than about 200 nanometers being referred to as the vacuum ultraviolet.
Most fibers, having a doped fused silica core and a fused silica cladding, are not however satisfactory for UV transmission since the dopants cause the fiber to absorb some components of UV light. As fused silica has the lowest UV absorption of glassy materials, it is potentially an extremely suitable material for a UV transmitting fiber, but its low index of refraction makes the finding of a suitable cladding difficult.
Certain polymers, notably siloxanes (polydimethyl siloxanes) and fluorocarbons such as FEP (fluorinated ethylene-propylene) do have a lower refractive index than fused silica, and are thus usable as claddings for silica cored fibers. Such fibers are generically referred to as PCS fibers. It is known that these fibers will transmit UV light, but they suffer from several disadvantages: (1) because the core-cladding interface is not as good as that in an all-silica fiber, scattering is more severe and this decreases transmission (this effect is more pronounced at shorter wavelengths), (2) siloxanes absorb substantially below 210 nm, and (3) the presence of a polymer cladding renders the fiber less suitable for operation at high temperatures or in other harsh environments.
A fiber having a transmission in the vacuum UV is thus desirable but not obtainable with present commercial PCS fibers.
Fibers comprising a core of fused silica and a cladding of fluorine- or boron-doped fused silica have been developed for their radiation hardness, since they contain substantially only low atomic weight elements. Such fibers, and methods for their manufacture, are disclosed in, inter alia, U.S. Pat. Nos. 4,082,420; 4,161,505; 4,162,908; 4,165,152; 4,165,915; 4,221,825; 4,242,375; and 4,295,869. Preforms for such fibers are generally produced by the combustion of a silicon compound in oxygen to produce synthetic fused silica, and the dopant is introduced by the addition to the combustion mixture of a source of the dopant, eg. BBr.sub.3 for boron or CCl.sub.2 F.sub.2 for fluorine. Combustion of the dopant sources in the oxygen produces the dopant material, and by fluorine-doped synthetic fused silica is meant a synthetic fused silica into which has been introduced fluorine or a chemical compound thereof.